This invention relates to continuously variable transmissions and, more particularly, it concerns a unique geometry for such transmissions by which such factors as the range of available speed ratios, reduction in overall transmission length, and adaptability to existing power train configurations is improved relative to prior transmissions of comparable performance.
U.S. Pat. Nos. Re. 29,328, Re. 30,981 and 4,112,780 exemplify several embodiments of a continuously variable, mechanical power transmission in which three frame-supported working bodies operate to transmit a mechanical power input to a rotatable output at infinitely variable output/input speed ratios within the design range of the transmission. For purposes of definition in this background discussion as well as in the ensuing detailed description of the present invention and in the appended claims, the three working bodies may be termed, respectively, an "alpha body" which is supported by the transmission frame to be concentric with a first or primary transmission axis, a "beta body" which is supported by the alpha body to be concentric with a second axis inclined with respect to and intersecting the first axis at a point of axes intersection, and an "omega body" carried by the frame to be concentric also with the first axis. Although any one of these three bodies may be rotatable on the respective axes with which they are concentric, one of the three is held against rotation to provide a reaction torque whereas the other two bodies are rotatable and coupled either directly or by gearing to the respective input and output shafting of the transmission.
It is to be noted that the terms "alpha body," "beta body" and "omega body" are completely arbitrary and as such, do not restrict the components designated thereby either to the class of transmission represented by the disclosure of to the aforementioned patents or to specific structure to be described hereinafter. The terms will, however, lend consistency of definition in the description to follow and facilitate an understanding of various speed relationships to be expressed by algebraic equations.
The continuously variable speed ratio capability of such transmissions is achieved by providing one of the beta and omega bodies with a pair of rolling or traction surfaces which are of revolution about the concentric body axis and which are of variable radii along that axis in symmetry with the point of first and second axes intersection. Physically, the rolling surfaces thus defined may be provided at the ends of a generally cylindrical beta body rolling against generally plate-like omega body members as in U.S. Pat. No. Re. 29,328, or one of the beta and omega bodies may be of biconical configuration and extend within the other of the beta and omega bodies, the latter being manifested as a pair of ring members as represented by several of the embodiments disclosed in U.S. Pat. No. Re. 30,981. The pairs of rolling surfaces on the beta and omega bodies are retained in frictional engagement with each other at two contact points or zones capable of positional adjustment to vary the ratio of the beta body surface radius (R.sub.b) to the omega body surface radius (R.sub.w). Thus, if the alpha body is rotatable at a velocity (.alpha.) about the first axis, the rotational speed of the beta body about the second axis in the (.alpha.) frame of reference is (.beta.) and the rotational speed of the omega body on the first axis is (.omega.), then the respective speeds of the three bodies are related by the following equation: EQU .omega.-.alpha.+.beta.* R.sub.b /R.sub.n =0. (1)
A generally preferred mode of operating such transmissions has been to apply an input torque to the alpha body to carry the beta body in nutation and hold the omega body against rotation (.omega.=0). The beta body is linked with an output shaft rotatable on the first axis by gearing having a ratio factor (k) which theoretically may be of any value and also may be made either positive or negative depending on the particular gearing arrangement used. In light of the foregoing, where .theta. is unit output speed and taking into account the gearing ratio (k), the output/input speed ratio of the unit is determined by an equation: EQU .theta./.alpha.=1-k.rho.. 2)
A principal advantage of operating in the mode represented by equation (2) is that the physical parameters of such transmissions readily accommodate a range of values for the function (K.rho.) which permit a continuously variable output/input speed ratio range of from zero to 1 (1.0&lt;k.rho.&gt;2.0). Also, this range may be shifted to include an output reversal through zero merely by selecting a gear ratio (k) so that the function (k.rho.) brackets a numerical value of 1 (e.g., 1.5&gt;k.rho.&gt;0.7).
A geometric characteristic common to all embodiments of traction drive transmissions represented by the disclosures of the cited patents is that the points of rolling friction contact, which are in a plane containing the first and second axis, always lie outside of the areas of that plane subtended by the first and second axes. This characteristic of the prior transmissions is due in part to the relatively small angle subtending the first and second axes but the size of that angle is not the sole contributing factor. The geometric variation available to rolling surface configuration is also a factor. Although a wide variety of rolling surface configurations may be used in the prior transmission embodiments, it is believed that the described geometric characteristic has the effect of limiting the range of available beta/omega surface radius ratios and thus the intrinsic speed ratio range of the transmission.
While traction drive transmissions of the class exemplified by the aforementioned patents have demonstrated a capacity for highly efficient operation in the transmission of power at any speed ratio within the range of the transmission design, the geometry of such transmissions is also restrictive to dimensional proportioning. For example, in embodiments involving circumferential enclosure of the beta and omega bodies, one within the other, speed ratio range is limited to variation of the radius ratio factor .rho. which can be achieved by axial movement of the omega rings along the surfaces of the beta cones. Increased speed ratio range can be accomplished either by additional gearing or by increasing the axial length of the transmission. While the increased length is not a problem to many drive line applications, it can be a serious impediment to use of the transmission particularly in automobile drive trains where space available for an engine/transmission assembly is limited. This is especially true of front-wheel drive power trains for automotive vehicles.